The present invention relates to an apparatus for producing images of an object and in particular for observing the rear portions of the eye.
Apparatuses for producing images of an object for observing the rear portions of the eye are well known in the art and are implemented with various equipment for producing images. By way of illustration, a number of laser-scanning cameras, laser-scanning microscopes and laser-scanning ophthalmoscopes have been proposed in medical technology.
Apparatuses of the known type have proven to be particularly advantageous when a comparatively large object has to be viewed through a small aperture arranged before the object. For example, when observing the rear portions of the eye, there arises the problem that the fundus must be illuminated and observed through the pupil of the eye and frequently through the not clear anterior media of the eye, where reflexes occur to produce image defects. Similar circumstances have been encountered in other cases of medical or technical application.
For this reason, fundus cameras have usually been employed in the past to observe the rear portions of the eye, in which the entrance pupil and the exit pupil were separated according to "GULLSTRAND" to suppress the so-called corneal reflex i.e. the part of the pupil of the eye used for the illumination surrounds ring-like the part used for observation.
Nonetheless, reflexes cannot be entirely suppressed when working with such fundus cameras. Moreover, the attainable resolution of approx. 15 .mu.m is often insufficient.
Therefore, it has repeatedly been proposed to employ apparatuses for observing the fundus of the eye, which do not illuminate expansive areas of the fundus of the eye, but rather scan as small as possible a spot with a focused illuminating light and detect the reflected light in relation to the scanning sequence. In this regard, reference is made, by way of illustration, to "The Foundations of Ophthalmology", Vol. VII, pp. 307/308, Yr. 1962, U.S. Pat. No. 4,213,678, EP-A-0 145 563 as well as Japanese patent publications 61-5730 and 50-138822.
The apparatuses described in the aforementioned references differ from one another in the pupil separation. The Japanese patent publication 61-5730 proposes a "GULLSTRAND separation", U.S. Pat. No. 4,213,678 an inverted "GULLSTRAND pupil," and the Japanese patent publication 50-138822 adjacent pupils as illumination and observation light.
In the apparatus for observing the rear portions of the eye described in EP-A-145 563, both the illumination light beam and the observation light beam are directed via a scanning device. A "double scanning system" of this type has the advantage that the reflected beam of light can be determined by means of a stationary detector with a relatively small surface.
The apparatuses mentioned in the preceding for observing the rear portions of the eye with "scanning illumination" have in common that the resolution of the received image is determined by the size of the "focus spot" (approx. 8-12 .mu.m) on the fundus of the eye and that the reflected light is received by a single detector with a more or less large field of view aperture for building up the image of the rear portions of the eye.
Further analysis of the returned light has not previously been considered.
The present invention is based on the recognition of the fact that apparatuses for producing images, in particular, due to special types of illumination and/or the analysis of the light scattered back according to most varied criteria permits gaining essential further ground-laying information about the object of which an image is to be made, for example the fundus of the eye, than is possible with any of the other known apparatuses.
Therefore, a primary object of the present invention is to improve an apparatus for producing images of an object hereto in such a fashion, that it is possible, by means of special types of illumination and/or the analysis of the light scattered back, to analyze the object to be imaged proceding beyond a pure image production analysis.
By way of illustration, the apparatus of the present invention permits making a spatial analysis and/or one of the polarization state of the returned light. For this purpose, the apparatus is provided with several individual detectors, which, in order to receive the light reflected from different planes and/or to receive the patchlike distribution of the intensity of the light and/or to determine the polarization state of the light conjugate to different planes or assigned to different areas of a plane or to which corresponding apertures or polarization filters are connected in a series.
In this manner, apparatus permits measuring the spatial distribution of the intensity and/or the polarization state of light reflected from the object to be imaged, by for example the rear portions of the eye. Moreover, the arrangement of the detectors in planes conjugate to the different planes of the object to be imaged makes a depth analysis of the object possible.
It is expressly made clear at this point that when in the case of the present invention it is said that the detectors have a specific form or are arranged at a specific location, it is not necessary that the detectors actually are designed accordingly; but rather it suffices if apertures for determining the field of view are arranged at the corresponding location and are connected with the detectors via light-conducting means, for example relay optics or light conductors. Such an arrangement of field-of-view-determining apertures instead of detectors is a particular feature of the "scan process" used for image build-up, in which no actual image is produced, but reflected or scattered light is received and assessed time-sequentially to the image build-up at any time in the entire space angle or the assessable space angle. Another primary object of the present invention is that light of several wavelengths, preferably the light from several lasers, is projected simultaneously onto the location of the object to be imaged. By this means varied effects can be achieved.
By way of illustration, it is possible to simulate a white light illumination in a laser scanning ophthalmoscope and deliver the ophthalmologist his "accustomed" picture of the fundus of the eye with a "representation in real color".
Moreover, with an appropriate selection of the wavelengths it is possible, by way of illustration, to determine the blood oxygen saturation level, which indicates local circulatory disturbances and of the entire system, avascular zones, etc. Furthermore, tumor analysis, visual pigment analysis, etc. are possible.
In addition, the simultaneous reception of an angiofluorescent and a "normal" image of the fundus of the eye is possible if an Ar.sup.+ -laser or a HeNe laser and a laser with a different wavelength are employed simultaneously.
Simultaneous implementation of light of several wavelengths, moreover, makes other interesting possibility feasible.
The depth of focus of the illumination light depends on the entrance pupil, i.e. on the size and form of the pupil for the illumination light. By way of illustration, a great depth of focus is gained with an entrance pupil designed as an inverted Gullstrand pupil as proposed in the U.S. Pat. No. 4,213,678 due to the small peripheral angle of the illumination light. On the other hand, a small depth of focus is gained when a normal Gullstrand pupil or the pupil used in EP-A-0 145 563 is employed due to the large angle enclosed by the peripheral rays.
Normally, the entrance pupil is selected corresponding to the respective intention, whereby the inverted Gullstrand pupil yields the best resolution, because the optically poorer peripheral regions of the eye are not used in it and thus the illumination light can be focused onto the smallest spot diameter.
When several sources of light are employed yielding light of different wavelengths, different entrance pupils can be used, whereby, it is particularly preferable if for light of one wavelengthan entrance pupil delivering a great depth of focus is employed and an entrance pupil delivering a small depth of focus is used for light of another wavelength. In this manner, it is possible to simultaneously receive a full overall image with high resolution and great depth of focus and a second image, which is "depth selective", whereby the selection of the different entrance and, if required, exit pupils occurs, for example, by means of a suitable wavelength selective coating of the so-called optical input coupling element (divider mirror), i.e. of the mirror separating the illumination and observation beam path. In this case, complementary pupils are received for the different wavelengths. Naturally, a different separation of the pupils by suitable means is also possible, such as by employing several divider mirrors in such a fashion that other pupil separations are also possible as complementary pupils.
The signals from the various detectors, e.g. an angiograph image and the "normal" image can be superimposed on a monitor or presented on several monitors.
In any case, the representation of both or several images in real time or following storage can be coupled. By "coupling" it meant the known state of the art operations in image processing, by way of illustration, very revealing images are received by means of "real time superimposition" of an angiograph image and a normal image. The individual direct images or those received after processing can, of course, be presented and/or received on several observation devices simultaneously, such as monitors.
In this case, it is particularly advantageous to design the apparatus of the present invention in such a fashion that both the illumination light and the reflected light are directed via the scanning device. With such an apparatus, a simple detection light signal, which does not change its position in space, is received in a simple manner behind the scanning device.
As already explained in detail, in accordance with the present invention the spatial distribution of the reflected light can be detected and assessed. For this purpose, it is, in particular, possible to arrange detectors or field-of-view determining shutters in planes which are not conjugate to the actual plane of the object.
By way of illustration, a detector arrangement or field-of-view determining shutter is provided in a plane conjugate to the pupil of the eye to detect the distribution of the intensity of the reflected light in this plane. In this case, the individual detectors or shutter elements preferably have the shape of sectors of a circle so that the main point of the reflected light or light scattered back, or directional symmetries etc. can be determined, by means of which, by way of illustration, it is possible to draw conclusions about the surface structures.
In a further embodiment, a detector arrangement or a shutter arrangement is provided in a plane conjugate to the object to be imaged, thus, by way of illustration, conjugate to the fundus of the eye, which determines the intensity of the distribution of the reflected light in this plane.
By this exemplary means, it is possible to detect the portion of transverse scattering in the retina by analyzing the distribution of the intensity in an image plane conjugate to the retina and thereby gain information about the structure of the retina.
In addition to the spatial analysis of the reflected light, another embodiment makes it possible to analyze the polarization state of the reflected light so that an improved representation of the double-refracting nerve fiber layer of the retina is yielded compared to that of the known apparatuses.
A further embodiment, moreover, permits determining the Stokes parameters to describe the polarization characteristics by which local defects can be detected, and anistropic, i.e. directional structures of the retina, such as, for example, the nerve fiber layer can be made more prominent. Moreover, with regard to the definition of Stokes parameters, reference is made to the article "Polarization Imaging" in APPLIED OPTICS; Vol. 20, pp. 1537 ff.
Further features of the present invention make an optical structure analysis or an optical image pre-processing of the reflected light possible. The optical image pre-processing operates substantially faster than available electronic image processing systems and thereby permits making special structures of objects prominent in real time even in the case of complex filtering.
Depending on the position of the analyzing filter or the shutter, polarization states can be isolated, directional anisotropes can be selected, aberrations can be compensated for, etc. It is particularly advantageous if variable filters and/or shutters controlled by, by way of illustration, a calculator exchangeable and/or rotatable shutters as well as shutters having a gradual transmission (apodization) are provided. Such shutters are generally not simple apertures, but rather formed arrangements depending on the respective use, by way of illustration, slits, rings, combinations of quadrants or a pattern of individually controlled dots. Furthermore, the shutters may be partially mirrored glass plates, perforated mirrors, wavelength-selectively vaporized mirrors or semi-translucent mirrors. It is also possible to produce zones of different optical effects by means of appropriate design of the divider mirror.
The present invention provides a simply constructed light modulator, namely, a liquid crystal element, which can be switched area-selectively translucent or untranslucent.
Another preferred solution for accomplishing a primary object of the present invention and, in particular, a design of an apparatus for observing the rear portions of the eye, is distinguished by the fact that, in addition, marks may be projected onto the object to be observed, by such as the fundus of the eye. These marks can, by way of illustration, serve to mark areas to be treated (coagulated) or to be examined, and are produced by means of image processing in a representation which was previously made. On this point, patent application P 36 07 721.6, discusses in detail the use of image processing for treatment planning and for producing markings.
These marks may, by way of illustration, be produced by "lightening" the illumination light beam at the corresponding location. The lightening is particularly preferred when the marks are to serve for orientation and/or marking a specific area as, in any case, the spatial relationship between the "scanning beam" and the mark remains constant.
However, it is also possible to employ a light source and in particular a positioning unit to project the marks as, in that case, far as possible independence of the individual systems of each other is ensured. This construction is especially recommended if the additional beam is intended for processing purposes, such as coagulating purposes.
This positioning system can, by for example, by an acousto-optical deflector or a wobble optic shown in German patent application P 35 32 464.3, as the same is stable and easy to handle.
On the other hand, in accordance with the present invention, an x/y-scanner with a polygonal mirror drum and a galvanometer mirror is preferably used as the scanning device for a system of this type which operates independently of wavelengths, and is particularly of advantage in the case of the simultaneous implementation of light of several wavelengths.
Besides the possible applications of the additional mirrored-in markings previously explained as a further possible application there is the fundus perimetry. In this case the markings serve as so-called stimuli, which the patient recognizes or in the event of field-of-view failures does not recognize. It is particularly advantageous if an infrared laser is employed as the source of the observation light as thereby the observation light does not have an adverse effect on the recognition of the stimuli. By means of this embodiment of an apparatus for observing the rear portions of the eye, a fundus perimeter is yielded that makes microperimetry possible under visual control and moreover also permits training the eye to eliminate vision or fixation weaknesses.
In microperimetry it is also advantageous, however, not only when the present invention is employed, if additional illumination of the surrounding area is provided, which, by way of illustration, is coupled via a semitranslucent mirror. This illumination of the surrounding area permits, for example, perimetry of a "specific level of lightness", but, of course, has other advantages.
In fundus perimetry, special search algorithms, scotoma detection, a variable measuring point density, fundus tracking for automatic detection of the position of the projected markings on the fundus can be realized via a control unit. Furthermore, a positive and negative perimetry, color differential examinations, etc. may be carried out.
A further improvement, in accordance with the present invention, of an apparatus, which is particularly suited for the observation of the rear portions of the eye, is provided with one or several detectors, before which shutters are arranged in such a manner that the result a dark field illumination of the detectors so that only the multiply scattered components are recorded in the image of the object or the fundus. By this means an intensified contrast representation of specific structures, e.g. papillary tissue, is made possible, which is of utmost interest for automatic determination of the edge of the papilla within the scope of glaucoma diagnosis.
By providing several detectors it is, of course, also possible to receive "dark-field" and "light-field" images simultaneously.
Furthermore, in accordance with the present invention it is advantageous if earlier received images and/or markings are mirrored congruently into the received and presented image in such a manner that the operating personnel, by way of illustration, can compare images made according to a different method, such as angiographs, or control an automatic laser beam positioning. To this point, reference is again made to the German patent application P 36 07 721.6. The "mirroring in" can occur by optical means, preferably, however, by electronic means in the observation apparatus, such as by means of a monitor.
The apparatus of the present invention is not only suited as an image-producing or diagnostic device, but also as a processing or therapy device and can be combined with many very different instruments, such as processing or treatment lasers of different wavelengths.
In particular, however, it is advantageous if in addition the beam of a coagulating laser, e.g. an Ar.sup.+ -laser or a dye laser, is mirrored in preferably between the scanning device and the eyeball. However, "a short" raising of the output of the observation laser is, of course, also possible for coagulating as described in U.S. Pat. No. 4,213,678. The joint implementation of the "scanning device" permits in this case in particular treating large areas or several areas in one step.
An apparatus for observing the rear portions of the eye with a scanning illumination is particularly useful as an image transmitter for a so-called eye tracking unit due to its reflex-free and high resolution presentation of the image. With regard to the principal eye-tracing-concept-directing of the observation and/or treatment unit, switching off the laser in the event of eye motion, etc. reference is again made to the German patent application P 36 07 721.6, the content of which, moreover, is expressly viewed as a disclosure of this application.
Furthermore, an apparatus for observing the rear portions of the eye with a scanning illumination is especially suited as an image transmitter for treatment planning due to its reflex-free and high resolution presentation of the image as also already described in the German patent application P 36 07 721.6.
It is especially advantageous, however, to employ an apparatus with "double scanning", in contrast to the apparatus known from EP-A-0 145 563, a field-of-view shutter having a diameter from 80 to 150 .mu.m is used, being thus substantially larger than the diameter of the point image on the fundus of the eye which is typically between 8 and 12 .mu.m and at the most, however, approx. 20 .mu.m. This selection of field-of-view shutter, which differs from the selection in EP-A-0 145 563, particularly supports the concept of the present invention of using several individual detectors. The pupil separation in the pupil plane may be carried out in a state of the art manner from the above-mentioned individual references.